This invention relates to communication systems and more particularly to a cryptographically secure communication system employed with a transmission medium having a low frequency pass band and relatively narrow bandwidth, such as a telephone transmission channel and the like.
There is no easy way to encrypt speech or any other analog waveform so that no information about the waveform is available from the encrypted version, and yet the decryption process recovers the waveform exactly. On the other hand, if the waveform is transmitted approximately, being represented as a sequence of discrete symbols selected from a finite alphabet, there are ways of encrypting that deny all information about the waveform to an observer of the transmitted symbols. Such a prior art arrangement of encrypting speech waveforms by this approximation process is described hereinbelow with respect to FIGS. 1, 2 and 3. This prior art arrangement, however, has the disadvantage of effectively amplifying noise introduced in the transmission medium and thereby reducing its effectiveness.